Nowadays, ethylene oxide is produced by catalytic gas phase oxidation of ethylene using a molecular oxygen-containing gas in the presence of a silver catalyst. An outline of a purifying method in a process for producing ethylene oxide is as follows (for example, refer to JP 62-103072 A).
First, ethylene and a molecular oxygen-containing gas are subjected to catalytic gas phase oxidation on a silver catalyst to obtain an ethylene oxide-containing reaction product gas (reaction step). Subsequently, the resulting reaction product gas is introduced into an ethylene oxide absorption column. The reaction product gas is brought into contact with an absorption liquid mainly containing water. Ethylene oxide is recovered as an aqueous solution (absorption step). Subsequently, the recovered ethylene oxide aqueous solution is fed to a purification system of ethylene oxide to obtain high-purity ethylene oxide through several stages. The ethylene oxide purification system usually includes a stripping step, a dehydration step, a light fraction separation step, a heavy fraction separation (purification) step, and the like.
Many steps in the process for producing ethylene oxide (for example, a heavy fraction separation (purification) step in a purification system) require thermal energy, and water vapor is mainly used as a supplying source thereof. Therefore, when a production amount of ethylene oxide is increased, an amount of water vapor as a thermal energy source is also increased. This increases running cost and reduces a profit.
In the related art, as a technology for recovering a thermal energy in a process for producing ethylene oxide, the following technology is proposed. That is, water vapor is generated using reaction heat generated in a reaction step to be used as a power source of a pump or the like, a driving source of a generator, or process steam of an ethylene oxide production plant and an ethylene glycol production plant (for example, refer to JP 2012-214399 A). As an example of recovering exhaust heat of an exhaust gas from a column top of a distillation column, a technology is known, in which exhaust heat of an exhaust gas from a column top of an ethylene oxide stripper column is recovered as a heat source of an ethylene oxide purification column (for example, refer to JP 63-30476 A).
By the way, in the related art, an exhaust gas containing unreacted ethylene discharged from a column top part of the ethylene oxide absorption column, a carbon dioxide gas (carbon dioxide; CO2) and water as by-products, and an inert gas (nitrogen, argon, methane, ethane, or the like) is circulated into an ethylene oxidation step as it is. Alternatively, a part thereof is extracted and introduced into a carbon dioxide gas absorption column, and the carbon dioxide gas is selectively absorbed by an alkali absorption liquid. The absorption liquid is supplied to a carbon dioxide gas stripper column to strip and recover the carbon dioxide gas (for example, refer to JP 60-131817 A). Further, a carbon dioxide gas-containing gas containing a carbon dioxide gas stripped and recovered in a carbon dioxide gas stripper column is usually discharged from a column top part of the carbon dioxide gas stripper column to be purged into the atmosphere (for example, refer to “Ethylene oxide, Ethylene glycol” written by Itsuo Tadasue, PETROTECH, Vol. 20, No. 3 (1997)).